The present invention relates to a gas sensing element capable of detecting emission gas such as NOx and preferably employable in an exhaust system for an internal combustion engine of an automotive vehicle.
Harmful gases emitted from automotive internal combustion engines cause air pollution as a serious problem the modern society now faces. Various laws and regulations require automotive manufacturers to satisfy severe standards for promoting emission gas purification. Under such circumferences, it is known that the emission gas purification can be effectively performed by directly detecting the NOx concentration to feedback control the engine combustion as well as to monitor the catalyst condition based on the detected NOx value.
FIGS. 9 and 10 show conventional gas sensing elements.
A pump cell 3 faces a first chamber 11. To perform pumping of oxygen between the first chamber 11 and the outside of the sensing element, a voltage is applied to the pump cell 3. A monitor cell 95 detects a concentration of oxygen in the first chamber 11. The pump cell 3 is feedback controlled based on a detected value of monitor cell 95 to maintain a constant oxygen concentration.
A sensor cell 2 faces a second chamber 12. The sensor cell 2 measures oxygen ions produced from NOx in the second chamber 12 and produces a sensor signal (i.e., oxygen ion current) representing a NOx concentration based on the measured oxygen ions. As the oxygen concentration in the second chamber 12 is constant, an amount of oxygen ions moving across the sensor cell 2 is proportional to the NOx concentration. In other words, the oxygen ion current of the sensor cell 2 is proportional to the NOx concentration.
Thus, the NOx concentration can be accurately measured irrespective of change of the oxygen concentration in the measured exhaust gas.
In this case, the sensor cell 2 is made of a material capably of decomposing NOx into oxygen ions and nitrogen ions to measure the NOx concentration. However, when the sensor cell 2 is made of other material, the sensor cell 2 will be able to measure another gas concentration.
However, the conventional gas sensing elements have the following problems.
The monitor cell, provided in the first chamber, cannot accurately monitor the oxygen concentration in the vicinity of the sensor cell provided in the second chamber. A significant difference will appear between the oxygen concentration of the first chamber and that of the second chamber.
The second chamber communicates with the first chamber via a narrow passage (i.e., diffusion resistive passage). Presence of such a narrow passage possibly delays transmission of oxygen concentration change to the second chamber compared with transmission to the first chamber. Accordingly, when the monitor cell is provided in the second chamber, the control of the first chamber is delayed. The response of control is worsened.
To solve the above-described problems, an object of the present invention is to provide a gas sensing element having excellent response and capable of accurately detecting a specific gas concentration in a measured gas irrespective of unpredictable or unstable distribution of oxygen gas concentration.
In order to accomplish the above and other related objects, the present invention provides a first gas sensing element comprising first and second chambers into which an objective gas to be measured is introduced. A first diffusion resistive passage connects the first chamber to an outside of the gas sensing element. A second diffusion resistive passage connects the first chamber to the second chamber. A pump cell, provided on a surface defining the first chamber, performs pumping of oxygen in accordance with an applied voltage. A first monitor cell, provided on a surface defining the first chamber, generates an electromotive force representing an oxygen concentration in the first chamber. A second monitor cell, provided on a surface defining the second chamber, generates an electromotive force representing an oxygen concentration in the second chamber. A sensor cell, provided on a surface defining the second chamber, is responsive to application of a predetermined voltage for generating a sensor current representing a specific gas concentration in the objective gas. And, the voltage applied to the pump cell is controlled based on the electromotive forces obtained from the first and second monitor cells.
According to the first gas sensing element, the first and second monitor cells face the first and second chambers respectively. The voltage applied to the pump cell is controlled based on the electromotive forces obtained from the first and second monitor cells.
The first gas sensing element of the present invention operates in the following manner.
The first monitor cell interposes between the first chamber and the reference gas chamber. The second monitor cell interposes between the second chamber and the reference gas chamber. Each of the first and second monitor cells generates an electromotive force in response to a measured oxygen concentration.
When the oxygen concentration in the measured gas is stable, there is no substantial difference between the oxygen concentration in the first chamber and the oxygen concentration in the second chamber. Thus, the electromotive force of the first monitor cell is substantially identical with that of the second monitor cell.
In this case, the voltage applied to the pump cell is controlled based on the electromotive force of the second monitor cell because the second monitor cell can accurately monitor the oxygen concentration in the vicinity of the sensor cell due to their positional relationship.
When the oxygen concentration in the measured gas is varying widely, the change of oxygen concentration is transmitted first to the first chamber and then transmitted with a larger delay to the second chamber. In other words, the electromotive force of the first monitor cell is apparently different from that of the second monitor cell.
When the oxygen concentration in the measured gas is increasing gradually, the electromotive force of the first monitor cell becomes smaller than that of the second monitor cell. On the other hand, when the oxygen concentration in the measured gas is decreasing gradually, the electromotive force of the first monitor cell becomes larger than that of the second monitor cell. This is due to time delay required when the measured gas passes through the diffusion resistive passage connecting the first chamber to the second chamber.
In such a transient state, to suppress adverse influence caused by deterioration in response, the voltage applied to the pump cell is controlled based on the electromotive force of the first monitor cell because the first monitor cell can promptly monitor the change of oxygen concentration caused in the measured gas.
As described above, the present invention provides the first gas sensing element which has excellent response and is capable of accurately detecting a specific gas concentration in a measured gas irrespective of unpredictable or unstable distribution of oxygen gas concentration.
Each of the sensor cell, the pump cell, and the monitor cell consists of a pair of electrodes with each electrode being made of a material individually selected considering the position where the cell is provided.
For example, the sensor cell has an electrode facing the second chamber. This electrode is required to have a function of generating oxygen ions from the specific gas to be detected.
The pump cell and the monitor cell have electrodes facing the first and second chambers. Preferably, these electrodes are inactive against the specific gas to be detected.
With this arrangement, it becomes possible to cause the decomposition of the specific gas in a limited region on the sensor cell, thereby enabling accurate measurement of specific gas concentration.
According to the first gas sensing element of the present invention, it is preferable that the first monitor cell and the second monitor cell are connected in parallel with each other.
This provides a simplified circuit arrangement for obtaining an average of the electromotive forces produced from the first and second monitor cells.
The present invention provides a second gas sensing element comprising first and second chambers into which an objective gas to be measured is introduced. A first diffusion resistive passage connects the first chamber to an outside of the gas sensing element. A second diffusion resistive passage connects the first chamber to the second chamber. A pump cell, provided on a surface defining the first chamber, performs pumping of oxygen in accordance with an applied voltage. A monitor cell is provided on either a surface defining the first chamber or a surface defining the second chamber. A sensor cell, provided on a surface defining the second chamber and responsive to application of a predetermined voltage, generates a sensor current representing a specific gas concentration in the objective gas. And, the voltage applied to the pump cell is controlled based on a limiting current obtained when a voltage is applied to the monitor cell.
The monitor cell is arranged to function as an oxygen concentration sensor in response to application of a voltage. The monitor cell produces an output current whose magnitude basically changes in accordance with an applied voltage but does not change in a specific voltage range irrespective of change of the applied voltage. The constant output current corresponding to the specific voltage range is generally referred to as a limiting current.
When the monitor cell faces the first chamber, the oxygen concentration in the first chamber is known from the limiting current value of the monitor cell. Thus, the oxygen concentration in the second chamber can be set to a lower constant value by controlling the voltage applied to the pump cell based on the limiting current value of the monitor cell. Furthermore, two-stage pumping is performed at the upstream of the sensor cell. Namely, pumping at the monitor cell and pumping at the pump cell are performed in the first chamber. This makes it possible to suppress the oxygen concentration dependency in the specific gas concentration detection.
Furthermore, as a voltage is applied to the monitor cell, the monitor cell is capable of pumping oxygen in the first chamber.
Therefore, when the oxygen concentration in the measured gas changes widely in a certain time period, variation of the oxygen concentration is followed up by the pumping function of the monitor cell. No problem will be caused due to delay in response.
When the monitor cell faces the second chamber, the oxygen concentration in the second chamber is known from the limiting current value of the monitor cell. Thus, the oxygen concentration in the second chamber can be set to a lower constant value by controlling the voltage applied to the pump cell based on the limiting current value of the monitor cell.
As the pump cell is controlled based on the oxygen concentration in the vicinity of the sensor cell, it becomes possible to accurately detect the specific gas concentration when the oxygen gas concentration is stable in a certain time period and there is a spatial distribution of oxygen concentration (i.e., when the oxygen concentration in the first chamber is different from that in the second chamber).
Furthermore, as a voltage is applied to the monitor cell, the monitor cell is capable of pumping oxygen in the second chamber.
Therefore, when the oxygen concentration in the measured gas changes widely in a certain time period, variation of oxygen concentration is followed up by the pumping function of monitor cell. No problem will be caused due to delay in response.
Accordingly, the second gas sensing element of the present invention makes it possible to accurately detect the specific gas concentration irrespective of change of oxygen gas concentration in the measured gas. Furthermore, according to the second gas sensing element of the present invention, the measured current of the monitor cell is utilized to control the pump cell.
This is effective to reduce error caused by offset current obtained when the specific gas concentration is 0, thereby realizing highly accurate detection of the specific gas concentration. The offset current is generally caused due to residual oxygen or leak current of each cell.
As described above, the present invention provides the second gas sensing element which has excellent response and is capable of accurately detecting a specific gas concentration in a measured gas irrespective of unpredictable or unstable distribution of oxygen gas concentration.
If an excessively high voltage is applied to the pump cell, there is the possibility that the specific gas may decompose even in a case where the electrode of the pump cell is a material inactive to the specific gas.
In view of the above, it is preferable to measure a current value of the pump cell and measure the oxygen concentration of the measured gas, and adjust the limiting current value in accordance with the measured value.
The present invention provides a third gas sensing element comprising first and second chambers into which an objective gas to be measured is introduced. A first diffusion resistive passage connects the first chamber to an outside of the gas sensing element. A second diffusion resistive passage connects the first chamber to the second chamber. A first pump cell, provided on a surface defining the first chamber, performs pumping of oxygen in accordance with an applied voltage. A second pump cell, provided on a surface defining the second chamber, performs pumping of oxygen in accordance with an applied voltage. A sensor cell, provided on a surface defining the second chamber and responsive to application of a predetermined voltage, generates a sensor current representing a specific gas concentration in the objective gas. A pump current is produced from at least one of the first and second pump cells in accordance with the pumping of oxygen. And, the pump current is utilized to control the voltage applied to the one of the first and second pump cells.
As described later with reference to FIG. 18, the pump cell current does not change in a predetermined voltage range of the applied voltage. This constant current value, i.e., the limiting current value, is dependent on the oxygen concentration.
Accordingly, the oxygen concentration in each chamber can be maintained at a constant value by adjusting the voltage applied to the pump cell in accordance with the pump current.
Furthermore, the third gas sensing element of the present invention comprises two pump cells which individually perform pumping in respective chambers. Thus, undesirable distribution of oxygen concentration will not appear in each chamber.
Moreover, even in a transient state where the oxygen concentration is widely changing, the response will not be so delayed because the pumping is independently performed in each chamber.
As described above, the present invention provides the third gas sensing element which has excellent response and is capable of accurately detecting a specific gas concentration in a measured gas irrespective of unpredictable or unstable distribution of oxygen gas concentration.
Furthermore, the third gas sensing element of the present invention does not require a monitor cell. Thus, the arrangement and control mechanism of the gas sensor can be simplified. Meanwhile, it is possible to provide a monitor cell as shown in a later-described fourth embodiment.
The present invention provides a fourth gas sensing element comprising first and second chambers into which an objective gas to be measured is introduced. A first diffusion resistive passage connects the first chamber to an outside of the gas sensing element. A second diffusion resistive passage connects the first chamber to the second chamber. A monitor cell is provided on at least one of a surface defining the first chamber or a surface defining the second chamber. A first pump cell, provided on a surface defining the first chamber, performs pumping of oxygen in accordance with an applied voltage. A second pump cell, provided on a surface defining the second chamber, performs pumping of oxygen in accordance with an applied voltage. A sensor cell, provided on a surface defining the second chamber and responsive to application of a predetermined voltage, generates a sensor current representing a specific gas concentration in the objective gas. And, the voltage applied to at least one of the first and second pump cells is controlled based on a limiting current obtained when a voltage is applied to the monitor cell.
According to the fourth gas sensing element of the present invention, the voltage applied to the pump cell can be controlled based on the limiting current obtained when a voltage is applied to the monitor cell. Thus, like the second gas sensing element, it becomes possible to reduce error caused by offset current obtained when the specific gas concentration is 0, thereby realizing highly accurate detection of the specific gas concentration.
Furthermore, the fourth gas sensing element of the present invention is based on a two-stage control of oxygen concentration using the first and second pump cells. Thus, like the third gas sensing element, the response will not be so delayed because the pumping is independently performed in each chamber.
As described above, the present invention provides the fourth gas sensing element which has excellent response and is capable of accurately detecting a specific gas concentration in a measured gas irrespective of unpredictable or unstable distribution of oxygen gas concentration.
According to the first to fourth gas sensing elements, it is preferable that each pump cell is provided on a surface defining a reference gas chamber.
This arrangement is preferably employed when the gas sensing element of the present invention detects a specific gas component involved in the exhaust gas of an internal combustion engine. More specifically, when the air-fuel ratio is shifted to a rich side, the measurement of the specific gas concentration is easily performed.
The gas sensing element of the present invention is applicable not only to a NOx sensor but also applicable to other types of gas sensors, such as a CO sensor, a CO2 sensor, a H2O sensor, a SOx sensor.